Lithography, generally called offset printing, easily achieves multicolor printing and is used for printing on paper, polyethylene terephthalate films, polyamide films, coated paper, etc.
In particular, general-purpose drying type offset inks have the composition shown in Tables 1 and 2 below.
TABLE 1 ______________________________________ Basic Composition of Offset Ink ______________________________________ ##STR1## ______________________________________
TABLE 2 ______________________________________ Composition of Offset Inks (parts by weight) For Sheet For Non- For Web Fed Press Paper Offset Press ______________________________________ Pigment 25 25 25 Resin 25 30 25 Drying oil 20 34 10 High-boiling 25 5 36 petroleum series solvent Drier 1 2 -- Others 4 4 4 Total 100 100 100 ______________________________________
The vehicle in Table 1 constitutes the liquid portion of printing ink, serving to disperse the pigment to fluidize the ink so that the ink may be transferred smoothly on a printing machine from an ink fountain to printing paper via rollers, a printing plate, and a blanket. The vehicle turns solid after printing, serving to fix the pigment on the printed surface. The vehicles are divided into a linseed oil type, an alkyl type, a quick-drying resin type, and a heat-set type.
It has recently been desired to reduce the drying time of offset ink to improve printing speed. To this effect, a quick-drying ink containing, as the vehicle, a drying oil compounded with a resin and a mineral oil (high-boiling petroleum series solvent) has been used.
However, when a polyolefin film or synthetic paper comprising a stretched polyolefin film containing an inorganic fine powder (cf. JP-B-46-40794 (the term "JP-B" as used herein means an "examined published Japanese patent application"), JP-A-61-279543 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), and JP-A-61-3748) is printed using the above-described quick-drying offset ink, the polyolefin becomes swollen with the vehicle contained in the quick-drying offset ink (particularly a high-boiling petroleum series solvent, such as mineral oil), and the printed film or synthetic paper suffers from so-called solvent attack, i.e., partial development of unevenness of the surface or curling of the whole film in case of solid printing. Hence, practical application of such quick-drying offset inks has involved difficulties.
Therefore, in actual offset printing on polyolefin films, a special offset ink designed for polyolefin films which contains no mineral oil is employed at the expense of its quick drying properties.
However, since such a special offset ink for polyolefin films requires a long drying time, and printing businesses and ink manufacturers capable of dealing with the special offset inks are limited, it has been desired to develop polyolefin films or synthetic paper on which general-purpose offset inks of the oxidative polymerization type (drying oil type) can be applied.
Since it is common for general printers to carry out offset printing of pulp paper, such as fine paper and coated paper, using commercially available quick-drying inks, they must substitute the quick-drying ink with the special offset ink for non-absorbing materials each time polyolefin film or synthetic paper is to be printed.
Considering the increased time and labor involved in the ink substitution, general printers are unwilling to undertake printing of polyolefin-based films such as synthetic paper. This has been one of the reasons preventing a broader application of polyolefin films or synthetic paper in offset printing.